An in-vehicle or in-car communication system is a feature offered on some vehicles, which uses existing hands-free audio equipment to provide what is essentially a small “public address” system between zones in the cabin of a vehicle. As shown in FIG. 1, a motor vehicle 102 having an interior or cabin space identified by reference numeral 104, enables rear seat passengers 110 and 112 to hear the passengers 106, 108 in the front of the vehicle 102. The speech from passengers 106, 108 in the front of the passenger compartment or interior 104 is “picked up” by microphones 114 and 116 that are coupled to an amplifier 118. The amplified speech is processed to remove noise and then provided to rear seat passengers 110 and 112 through one or more loudspeakers 126.
More specifically, audio frequency signals picked up by the microphones 114 and 116 are provided to a single or multi-microphone amplifier 118. The audio-frequency signals from the amplifier 118 are provided to a noise suppression processor 120. An echo canceller 122 removes audio frequencies detected by the microphones 114 and 116 that were output from one of the loudspeakers 126 located inside the vehicle's interior 104. A gain controllable amplifier 124 amplifies or magnifies the noise suppressed and echo cancelled audio from the microphone amplifier 118 and provides those signals to the in-vehicle speakers 126.
A problem with the prior art in-vehicle communications systems, including the system 100 shown in FIG. 1, is the oscillation or “howling” that inevitably occurs as the gain of the amplifier 124 increases, and as the acoustic characteristics of the vehicle's interior 104 change. By way of example, the acoustic feedback provided between the speakers 126 and the microphones 114 and 116 will change as the contents of the vehicle's interior 104 changes. Clothing, packages, and even the number of occupants, will change the acoustic characteristics of the in-vehicle interior 104 and thus change the feedback of audio from the speakers 126 to the microphones 114 and 116.
In the prior art, the echo suppression 122 selectively reduces the gain or amplification provided by the amplifier 124 but only for relatively narrow audio frequency bands. Such “notch” filtering causes the speech produced output from the speakers 126 to be distorted and is sometimes unintelligible. The echo canceller removes echo over a broad band, but not enough in high volume conditions to prevent howling. An apparatus and method for preventing howling before it occurs would be an improvement over the prior art and would produce enhanced audio fidelity in an in-vehicle audio communications system.